This present invention relates to the use of catalysts, which have an active component on a catalyst support containing silicon dioxide, for catalytic reactions under hydrothermal conditions. Hydrothermal conditions prevail in chemical reactions in aqueous systems when the temperature is above the boiling point of water and pressure is above standard pressure.
A typical reaction under hydrothermal conditions is the hydration of olefins to yield the corresponding alcohols in the presence of phosphoric acid as the catalyst or active component on a catalyst support containing silicon dioxide.
Such a process is described, for example, in EP 0 578 441 A2. In that process, water and ethylene are reacted at temperatures of between 225 and 280.degree. C. and pressures of between 20 and 240 bar to yield ethanol. A molar ratio of water to ethylene in the range from 0.15 to 0.5 is used in this reaction. Catalyst loading, measured in grams of water/ethylene mixture per minute per milliliter of catalyst, may be selected within the range from 0.01 to 0.1 g/(min.times.ml). Diethyl ether is formed as a secondary product in that reaction. Isopropanol is produced by hydrating propylene under similar conditions to those stated above, but at a slightly lower temperature in the range between 180 and 225.degree. C. The byproduct in this reaction is n-propanol.
The catalyst support used for the active component phosphoric acid in EP 0 578 441 A2 comprises pellets of synthetic silicon dioxide having a high crush strength, high porosity and few metallic contaminants. The purpose of the pores of the support is to accommodate the active component. Pore volume is thus preferably greater than 0.8 ml/g. Average pore radius prior to use in the hydration process is in the range between 1 and 50 nm.
In order to achieve optimum hydration performance, EP 0 578 441 A2 specifies a silicon dioxide content of the support of at least 99 wt. % with below 1 wt. %, preferably below 0.3 wt. % of contaminants.
Catalysts undergo aging during operation, which is discernible by a reduction in activity and/or selectivity. Deactivation is frequently due to a reduction in the specific surface area of the support brought about by elevated temperatures. Specific surface area in the context of this application means the BET surface according to Brunauer, Emmett and Teller determined by nitrogen adsorption according to DIN 66 132.
The specific surface area of a support is closely related to its pore structure. Moreover, solids having a high surface area usually have a completely or predominantly amorphous structure, which has a strong tendency to take on a thermodynamically stable state by crystallite growth accompanied by a reduction in specific surface area.
It has been found that catalyst supports containing silicon dioxide are also subject to such aging. Hydrothermal conditions accelerate aging. It is furthermore known that contaminants, in particular alkali metals, promote the aging of supports containing silicon dioxide under hydrothermal conditions (c.f. for example R. K. Iler in The Chemistry of Silica, page 544, John Wiley & Sons (1979)).
It has surprisingly also been found that the catalyst supports based on pyrogenically produced silicon dioxide described in EP 0 393 356 are also subject to aging under hydrothermal conditions, wherein small pores combine to yield larger pores with loss of specific surface area. Initially, pore volume remains virtually unchanged during such aging. This aging is unexpected because the pyrogenic silicon dioxide of which the supports consist has excellent temperature resistance according to investigations with a scanning electron microscope, the morphology of pyrogenic silicon dioxide does not change on heating to temperatures of up to 1000.degree. C. for a period of 7 days (Schriftenreihe Pigmente Nr. 11: Grundlage von AEROSIL.RTM.; Degussa publication, 5th edition, June 1993, page 20).
An object of the present invention is accordingly to provide catalyst supports containing silicon dioxide which exhibit improved aging resistance when used under hydrothermal conditions.